In order to reduce the bandwidth required to transmit digital video signals, it is well known to use video compression whereby the data rate of a video signal is substantially reduced.
One of the most widely used video compression techniques is known as the MPEG-2 (Motion Picture Expert Group). MPEG-2 is a block based compression scheme, wherein a frame is divided into a plurality of blocks each comprising eight vertical and eight horizontal pixels. For compression of luminance data, each block is individually compressed using a Discrete Cosine Transform (DCT) followed by quantization, which reduces a significant number of the transformed data values to zero. For compression of chrominance data 1, 2 or even 4 blocks per colour difference signal are used per macroblock, depending on the colour sub-sampling mode. Frames based only on intra-frame compression are known as Intra Frames (I-Frames).
In addition to intra-frame compression, MPEG-2 uses inter-frame compression to further reduce the data rate. Inter-frame compression includes generation of predicted frames (P-frames) based on previous I-frames or P-frames. In addition, I and P frames are typically interposed by Bidirectional predicted frames (B-frames), wherein compression is achieved by only transmitting the differences between the B-frame and surrounding I- and P-frames frames or surrounding P frames). In addition, MPEG-2 uses motion estimation wherein the image of blocks of one frame found in subsequent frames at different positions are communicated simply by use of a motion vector.
As a result of these compression techniques, video signals of standard TV studio broadcast quality level can be transmitted at data rates around 2-4 Mbps.
A frequently performed operation by a user of video equipment is navigation through a video sequence to locate specific occurrence. Current analogue Video Cassette Recorders have fast search modes and slow motion to support the user in finding a specific occurrence. In digital storage devices, navigation becomes possible based on mosaic screens, wherein a picture contains a number of sub-pictures that have a temporal distance. In this way, the user can get an overview of a specific part of a video sequence, and by scrolling through the mosaic screen, specific sections of the video sequence can be selected. The mosaic screen may comprise sub-pictures with a constant interval or may relate to specific events such as a scene change. Specifically, sub-pictures and mosaic screens may be used in a hierarchical fashion to build up a Visual Table Of Content (VTOC) which significantly facilitates the navigation through video material.
The VTOC could be generated during recording or playback of the video material. At regular intervals or at specific occurrences, the current video image is sampled and stored. The image is then reduced in size and resolution to fit a sub-picture. The mosaic screen generated from a plurality of these sub-images is uncompressed and therefore is very large and requires a large bandwidth and storage capacity. Consequently, the mosaic screen is compressed using the MPEG-2 compression algorithm.
When a sub-picture of a mosaic screen is reused in a different mosaic screen, the compressed mosaic screen is de-compressed, the appropriate sub-picture(s) is/are selected and combined with new sub-pictures.
It is clear, that manipulation of compressed sub-pictures in compressed video signals is very complex and requires significant amount of processing power. Thus, manipulation becomes very slow and further requires extra power consumption of the processing resources required. Hence, an improved method of manipulation of sub-pictures in a compressed video signal domain would be advantageous.